Callisto is the outermost of the Galilean
moons of Jupiter. Recent discoveries by the Galileo probe suggest
that a saltwater sea may exist beneath the rock and ice surface.
This raises some interesting questions about the possibilities of life
on Callisto. Where there is liquid water, life can exist.
After Europa, Callisto may be the best candidate for extraterrestrial
life in the Solar System.

Evidence for there being such an ocean comes
from fluctuations in the moon's magnetic field which match the rotation
of Jupiter. The most likely explanation for this is
that Jupiter's magnetic field, which rotates with the planet, induces
electrical currents within Callisto which in turn creates its own magnetic
field.

Callisto displays signs of ancient and sustained
bombardment with space debris - asteroids, meteorites and comets - and
is in fact the most heavily-cratered object in the entire Solar system.
It displays no sign of hot or cold volcanism, as do its sister Galilean
moons - Ganymede, Europa and Io - and it is something of a mystery as
to the source of the heating that maintains the suspected sub-surface
ocean in a liquid state. However residual heat from radioactive
decay in Callisto's rocks, or less likely heating from the limited tidal
actions associated with Jupiter and the other Galilean moons, may be sufficient
to maintain a reasonable core temperature. In essence Callisto may
work the same way as Europa is thought to, but the energy input will be
very much lower. The variations in the tidal pull that Callisto
undergoes as it orbits Jupiter are tiny compared with the forces that
the inner moons - Io and Europa - experience.

A liquid saltwater ocean is one of the main
prerequisites for life, and planetary scientists are now eager to explore
Callisto at close quarters to investigate the possibilities.